The present invention relates to the radiographic arts. It finds particular application in the conjunction with forming of rotating anodes found in x-ray tubes for use with CT scanners and will be described with particular reference thereto. It should be appreciated, however, that the invention may also find application in other x-ray medical and non-medical devices, and the like.
A high power x-ray tube typically includes a thermionic filament cathode and a rotating anode which are encased in an evacuated envelope. A heating current, commonly of the order of 2-5 amps is applied through the filament to create a surrounding electron cloud. A high potential, of the order of 100-200 kilovolts, is applied between the filament cathode and the anode to accelerate the electrons from the cloud towards an anode target area. The electron beam impinges on a small area of the anode, or target area, with sufficient energy to generate x-rays. The acceleration of electrons causes a tube or anode current of the order of 5-200 milliamps. Only a small fraction of the energy of the electron beam is converted into x-rays, the majority of the energy being converted to heat.
To inhibit the target area from overheating, the anode rotates at high speeds during x-ray generation. The electron beam does not dwell on the small impingement spot of the anode long enough to cause thermal deformation. The diameter of the anode is sufficiently large that in one rotation of the anode, each spot on the anode that was heated by the electron beam has substantially cooled before returning to be reheated by the electron beam. Larger diameter tubes have larger circumferences, hence provide greater thermal loading.
The anodes are formed from a refractory material, such as an alloy of titanium, zinc and molybdenum, with an outer ring in the target area of tungsten or a tungsten rhodium alloy. The materials for the anode are compressed, in powder form, into an annular mold and sintered in a hydrogen atmosphere to form a solidified body about 1 cm thick and about 10 cm in diameter. The body contains numerous pores. These must be removed before the anode is used in the x-ray tube to prevent the introduction of gases into the envelope. The vacuum conditions are such as to cause slow outgassing from the pores, which is detrimental to the operation of the tube. Additionally, defects in the surface of the anode can lead to eccentricities in the rotation of the anode and poor quality of the x-ray beam.
Accordingly, the sintered body is conventionally heated to a temperature of around 800.degree. C. and pressed in a forge. The force required to compress the body to the density required for x-ray anodes is considerable. For a standard 10 cm anode, a force of about 200,000 tons is used. The force required increases with the square of the anode radius.
Recently, demands have been made for larger and larger x-ray anodes. Anodes of 20 cm or larger would be beneficial for certain applications. Currently, the maximum size of the anode is limited by the capabilities of the forge and the pressures which it is able to apply. There remains a need for a method of forming anodes of these larger dimensions.
In a number of industries, chemical high explosives have been used for shaping, welding, and cladding metals. High explosive forming has been carried out in one of two methods. In the standoff method, an explosive charge is located at some predetermined distance from the blank or shape to be formed. Water is generally used as a transfer medium for uniform transmission of energy from the explosion to the workpiece and to muffle the sound of the blast. In the "contact forming" method, the explosive charge is held in intimate contact with the workpiece.
Interface pressures acting on the workpiece can be a million or more kilograms per square centimeter, resulting in rapid shaping of the metal. However, stress waves tend to be induced in the metal which result in displacement, deformation, and possible fracture. Such uncontrolled explosive techniques do not guarantee a highly uniform target area suitable for x-ray anodes.
Techniques developed in the thermonuclear industry in the area of complex shaped explosive charges for initiating the fission of plutonium spheres have the ability to provide a controlled explosion. The present invention adapts these techniques to the compression of x-ray anodes.
The present invention provides a new and improved method of forming x-ray anodes which overcomes the above referenced problems and others.